Simultaneous multibeam approach for cancelling multiple mainlobe jammers while preserving monopulse angle estimation accuracy on mainlobe targets

Abstract

An improvement in monopulse radar achieves nulling of multiple mainlobe jammers while maintaining the angle measurement accuracy of the monopulse ratio by using multiple simultaneous beams, thereby obtaining the additional degrees of freedom necessary for cancelling more than one mainlobe jammer (MLJ). The beams are placed one null beamwidth apart in order to maintain orthogonality. The MLJs are nulled in the orthogonal direction of the angle estimate giving undistorted monopulse ratios.

We claim: 
     
       1. In a radar system for receiving, at a phased array or digital beamforming high-gain main antenna coupled to a receiver, received signals which may be made up of target signals, a multiplicity of mainlobe jammer signals, and noise, said system further including a monopulse processor for determining angle of arrival of said target signals from signals representative of formed sum and difference beams, the method of operation comprising the steps of: forming signals representative of three multiple simultaneous received product beam sets (a) a first one of said product beam set comprising a nominal beam set including a Σ 1  sum beam equal to the product of an elevation sum beam multiplied by an azimuth sum beam, a Δ A1  difference beam equal to the product of an elevation sum beam multiplied by an azimuth difference beam, a Δ E1  difference beam equal to the product of an elevation difference beam multiplied by an azimuth sum beam, and a Δ.sub.Δ difference beam equal to the product of an elevation difference beam multiplied by an azimuth difference beam, said nominal beam set being aimed in a direction toward said target,   (b) a second one of said product beam sets including a Σ 2  sum beam equal to the product of an elevation sum beam multiplied by an azimuth sum beam, and a Δ A2  difference beam equal to the product of an elevation sum beam multiplied by an azimuth difference beam, said second beam set being aimed in a direction one null beamwidth away from said nominal beam set along elevation, and   (c) a third one of said product beam sets including a Σ 3  sum beam equal to the product of an elevation sum beam multiplied by an azimuth sum beam, and a Δ A3  difference beam equal to the product of an elevation sum beam multiplied by an azimuth difference beam, said third beam set being aimed in a direction one null beamwidth away from said nominal beam set along azimuth;     nulling said mainlobe jammers by adaptively weighting and combining said signals representing said product beams to create signals representative of said formed beams with undistorted monopulse ratios for azimuth and elevation; and   determining the angle of arrival of said target signals from said undistorted monopulse ratios.   
     
     
       2. The method of claim 1, wherein said received signals further include sidelobe jammer signals, said system further including an auxiliary array of low-gain sensor elements and a multiple sidelobe canceller, said method comprising the further steps of: maintaining said mainlobes while said multiple sidelobe canceller is operable; and   combining the output signals of said multiple sidelobe canceller with said signals representative of multiple simultaneous received product beam sets.   
     
     
       3. The method of claim 1, wherein said received signals further include multiple jammer signals, said system further including an adaptive array, said adaptive array comprising multiple elemental sensors, and operating by preprocessing, to form an identical set of nulls responsive to said sidelobe jammers, comprising the further steps of: maintaining said main lobes while said adaptive array is operable;   spatially filtering said mainlobe jammer signals from signals supplied by said adaptive array to form spatially filtered output signals; and   combining said spatially filtered output signals with said signals representative of said multiple simultaneous received product beams.   
     
     
       4. The method of claim 1, wherein said multiplicity of mainlobe jammers is two in number, and wherein said step of forming simultaneous received product beams includes the step of forming said product beams to represent   Σ.sub.1 (T.sub.x, T.sub.y)=Σ.sub.a (T.sub.x)Σ.sub.e (T.sub.y)       Δ.sub.A1 (T.sub.x,T.sub.y)=Δ.sub.a (T.sub.x)Σ.sub.e (T.sub.y)       Δ.sub.E1 (T.sub.x,T.sub.y)=Σ.sub.a (T.sub.x)Δ.sub.e (T.sub.y)       Δ.sub.Δ1 (T.sub.x, T.sub.y)=Δ.sub.a (T.sub.x)Δ.sub.e (T.sub.y)       Σ.sub.2 (T.sub.x,T.sub.y)=Σ.sub.a (T.sub.x)Σ.sub.e (T.sub.y -T.sub.y0)       Δ.sub.A2 (T.sub.x,T.sub.y)=Δ.sub.a (T.sub.x)Σ.sub.e (T.sub.y -T.sub.y0)       Σ.sub.3 (T.sub.x,T.sub.y)=Σ.sub.a (T.sub.x -T.sub.x0)Σ.sub.e (T.sub.y)       Δ.sub.E3 (T.sub.x,T.sub.y)=Σ.sub.a (T.sub.x -T.sub.x0)Δ.sub.e (T.sub.y)     where Σ 1  has a symmetrical profile with respect to both the azimuth and elevation directions with maximum gain at the bore sight, Δ A1  has a symmetrical profile with respect to the elevation direction but is anti-symmetrical with respect to the azimuth direction, Δ E1  is symmetrical with respect to the azimuth direction but is anti-symmetrical with respect to the elevation direction, Δ.sub.Δ1 is anti-symmetrical with respect to both directions and has a zero response at the boresight, Σ 2  and Δ are steered to T y0  where T y0  is one null beam width away from the boresight along the T y  direction, Σ 3  and Δ E3  are steered to T x0  where T x0  is on null beam width away from the boresight along the T x  direction, and said formed beams are represented by     Σ.sub.a =Σ.sub.1 -w.sub.1 Δ.sub.E1 -w.sub.2 Σ.sub.2       Δ.sub.A =Δ.sub.A1 -w.sub.1 Δ.sub.Δ1 -w.sub.2 Δ.sub.A2       Σ.sub.E =Σ.sub.1 -w.sub.3 Δ.sub.A1 -w.sub.4 Σ.sub.3       Δ.sub.E =Δ.sub.E1 -w.sub.3 Δ.sub.Δ1 -w.sub.4 Δ.sub.E3     where {w 1  } are the optimal adaptive weights for suppressing said mainlobe jammers, said optimal adaptive weights being determined by conventional correlation processing.